Politicians on all sides in Northern Ireland have condemned the murder of a prison officer shot dead on a motorway as he drove to work.

The officer, who had more than 30 years' service, was ambushed on a stretch of the M1 between Portadown and Lurgan, Co Armagh on Thursday morning. One unconfirmed report claimed a gunman opened up when another car pulled up alongside.

It is understood the officer's car careered off the road when he was fired on and he died later.

Although no organisation has admitted responsibility, security chiefs believe republicans opposed to the peace process were involved.

The extremists have been involved in long-running protests against jail conditions inside Maghaberry.

A car understood to have been used in the attack was later found burned out in Lurgan where supporters of dissidents have backed the jail protest campaign.

"At this time, our thoughts and prayers are with the bereaved family and we condemn this murder in the strongest possible terms," they said in a joint statement.

"There can be no justification for this brutal attack as this man was going about his daily life. People who work for the Prison Service play a crucial role in our community and any attack on them is an attack on all of us.

"Actions like this have no place in society and those who carried out this murder have nothing positive to contribute, and we refuse to let the people behind this attack divert us from building a better and peaceful future for everyone.

"We appeal to anyone with any information on this murder to contact the PSNI."

Stormont justice minister David Ford said: "I want to express my deepest sympathy to the family and colleagues of the murdered prison officer.

"This is a horrific murder, for which there can be no justification, and will be rightly condemned by all across Northern Ireland.

"At its very core this is a human tragedy. A man has lost his life this morning and a family are grieving the loss of their loved one. My thoughts are also with the wider Prison Service family."

Jeffrey Donaldson, MP for Lagan Valley, said: "This is a terrible tragedy. Once again, a small minority are trying to drag us back to the dark days."

The shooting happened at around 7.30am, at the same time as a major security alert further along the motorway at a shopping centre at Sprucefield, near Lisburn, where bomb disposal experts were called in to check a car.

It is believed this vehicle might have been linked in some way to the shooting, according to some sources.

On Wednesday, Northern Ireland secretary Theresa Villiers warned that the threat from dissident republicans in Northern Ireland remained severe.

That came a week after the threat of attack in Great Britain was downgraded to moderate.

Sinn Fein Assembly member John O'Dowd said the shooting was "totally wrong".

"There is no justification for continued conflict in this society," he said.

Mr O'Dowd insisted the extremists would not be allowed to take the region backwards.

"We, as a society, have decided we want to move beyond them. The road they are on is not going to achieve anything."

The man is the 30th prison officer killed in Northern Ireland since 1974.

The dissident protests in Maghaberry have taken place at a time when the NI Prison Service is undergoing a turbulent programme of reform.

Thursday's incident represents a major blow for recently-appointed director general Sue McAllister, 51, from South Yorkshire, who took over the reins of the organisation earlier this summer.